PPTC material with low percolation threshold for conductive filler

ABSTRACT

A polymeric positive temperature coefficient (PPTC) device including a PPTC body, a first electrode disposed on a first side of the PPTC body, and a second electrode disposed on a second side of the PPTC body, wherein the PPTC body is formed of a PPTC material that includes a polymer matrix and a conductive filler, wherein the conductive filler defines 20%-39% by volume of the PPTC material.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/557,336, filed Sep. 12, 2017, the entirety of whichis incorporated by reference herein.

FIELD OF THE DISCLOSURE

Embodiments relate to the field of circuit protection devices, includingfuse devices.

BACKGROUND OF THE DISCLOSURE

Polymer positive temperature coefficient (PPTC) devices may be used asovercurrent or over-temperature protection devices, as well as currentor temperature sensors, among various applications. In overcurrent orover-temperature protection applications, a PPTC device act as aresettable fuse, designed to exhibit low resistance when operating underpredetermined conditions, such as low current. The resistance of thePPTC device may be altered by direct heating due to temperatureincreases in the environment of the PPTC device, or via resistiveheating generated by electrical current passing through the PPTC device.For example, a PPTC device may include a composite PPTC material formedof a polymer material and a conductive filler, wherein the PPTC materialtransitions from a low resistance state to a high resistance state dueto thermally-induced changes in the polymer material, such as a meltingtransition or a glass transition. At a transition temperature, sometimescalled a “trip temperature,” where the trip temperature may range fromroom temperature to well above room temperature, the polymer materialmay expand and disrupt the electrically conductive network of conductivefiller particles in the PPTC material, rendering the PPTC material muchless electrically conductive. This change in resistance imparts afuse-like character to PPTC materials, which resistance may bereversible when the PPTC material cools back to room temperature.

The cost and weight of a PPTC material are generally dictated by theamount (e.g., percent by volume) of conductive filler in the PPTCmaterial. In almost all applications, it is desirable to minimize thecost and weight of PPTC devices while maintaining desired operationalcharacteristics such as trip temperature. It is with respect to theseand other considerations that the present disclosure is provided.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended asan aid in determining the scope of the claimed subject matter.

A PPTC device in accordance with an exemplary embodiment of the presentdisclosure may include a PPTC body, a first electrode disposed on afirst side of the PPTC body, and a second electrode disposed on a secondside of the PPTC body, wherein the PPTC body is formed of a PPTCmaterial that includes a polymer matrix and a conductive filler, whereinthe conductive filler defines 20%-39% by volume of the PPTC material.

Another PPTC device in accordance with an exemplary embodiment of thepresent disclosure may include a PPTC body, first and second metallicfoil layers disposed on opposing sides of the PPTC body, respectively,and extending from first and second metallic traces at opposing ends ofthe PPTC body, respectively, wherein the first metallic foil layerextends toward, but does not contact, the second metallic trace, andwherein the second metallic foil layer extends toward, but does notcontact, the first metallic trace. The PPTC device may further includeelectrically insulating insulation layers covering the first and secondmetallic foil layers, and metallic electrodes disposed on the insulationlayers in electrical contact with the metallic traces. The PPTC body maybe formed of a PPTC material that includes a polymer matrix and aconductive filler, wherein the conductive filler defines 20%-39% byvolume of the PPTC material.

A PPTC material in accordance with an exemplary embodiment of thepresent disclosure may include a polymer matrix and a conductive filler,wherein the conductive filler defines 20%-39% by volume of the PPTCmaterial and is formed of particles having a median diameter of 50nanometers to 20 micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B illustrate a PPTC device according to embodiments ofthe present disclosure;

FIG. 2A and FIG. 2B illustrate the effect of varying the size ofconductive particles in PPTC materials on the resistivity andpercolation threshold of such materials;

FIG. 3 illustrates exemplary resistance behavior for a PPTC materialaccording to embodiments of the present disclosure;

FIG. 4 illustrates a PPTC device according to an embodiment of thepresent disclosure; and

FIGS. 5A and 5B illustrate PPTC devices according to various additionalembodiments of the present disclosure.

DETAILED DESCRIPTION

The present embodiments will now be described more fully hereinafterwith reference to the accompanying drawings, in which exemplaryembodiments are shown. The embodiments are not to be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided certain exemplary aspects of the present disclosure tothose skilled in the art. In the drawings, like numbers refer to likeelements throughout.

In the following description and/or claims, the terms “on,” “overlying,”“disposed on,” and “over” may be used in the following description andclaims. “On,” “overlying,” “disposed on,” and “over” may be used toindicate that two or more elements are in direct physical contact withone another. Also, the terms “on,” “overlying,” “disposed on,” and“over”, may mean that two or more elements are not in direct contactwith one another. For example, “over” may mean that one element is aboveanother element while not contacting one another and may have anotherelement or elements in between the two elements. Furthermore, the term“and/or” may mean “and,” it may mean “or,” it may mean “exclusive-or,”it may mean “one,” it may mean “some, but not all,” it may mean“neither,” and/or it may mean “both,” although the scope of the claimedsubject matter is not limited in this respect.

In various embodiments, novel device structures and materials areprovided for forming a PPTC device, where the PPTC device includes aPPTC material having a relatively low “percolation threshold,” where“percolation threshold” is defined as a minimum percentage by volume ofconductive ceramic filler in the PPTC material that is necessary forachieving a desired resistivity. In one example, a PPTC material inaccordance with the present disclosure may exhibit a resistivity ofabout 0.15 ohm-cm with a percolation threshold in a range of 20%-39%.

In various embodiments, a PPTC device may be constructed as shown inFIG. 1A and FIG. 1B. FIG. 1A illustrates a side cross-sectional view ofa PPTC device 100, where a PPTC body 104 is disposed between a firstelectrode 102 and a second electrode 106 that are arranged on a firstside and a second side of the PPTC body 104, respectively. FIG. 1Billustrates a configuration of the PPTC device 100 after a firstterminal 108 is joined to the first electrode 102 and a second terminal110 is joined to the second electrode 106. The first terminal 108 may bejoined to the first electrode 102 using any suitable, electricallyconductive means of affixation (e.g., by soldering, welding, conductiveepoxy, etc.) to form a first interface 112, and the second terminal 110may be similarly joined to second electrode 106 to form a secondinterface 114.

According to embodiments of the present disclosure, the PPTC body 104may be formed from of a PPTC material having a relatively lowpercolation threshold as further detailed below. The first electrode 102and the second electrode 106 may be formed of various metals, including,but not limited to, copper foil. In some embodiments, the copper foilmay be nickel plated. The first terminal 108 and the second terminal 110may also be formed of various materials, including, but not limited to,copper or brass. The embodiments are not limited in this context.

In some embodiments of the present disclosure, the PPTC body 104 may beformed of a composite PPTC material that includes a polymer matrix and aconductive filler. The polymer matrix may be, or may include, asemi-crystalline polymer such as a polyvinylidene fluoride (PVDF)polymer, an ethylene vinyl acetate (EVA) polymer, a high-densitypolyethylene (HDPE) polymer, an ethylene tetrafluoroethylene (ETFE)polymer, or a perfluoroalkoxy (PFA) polymer. The embodiments are notlimited in this context.

According to some embodiments of the present disclosure, the conductivefiller of the PPTC material may be formed of particles of anelectrically conductive ceramic material, including, but not limited to,titanium carbide, tungsten carbide, vanadium carbide, zirconium carbide,niobium carbide tantalum carbide, molybdenum carbide, titanium boride,vanadium boride, zirconium boride, niobium boride, molybdenum boride,hafnium boride, or mixtures thereof.

The percolation threshold of the PPTC material may be in a range ofabout 20% to about 39%. That is, the volume fraction of conductivefiller in the PPTC material may range from about 20% to about 39%. Thoseof ordinary skill in the art will recognize that a percolation thresholdin the stated range is lower than the percolation thresholds ofconventional PPTC materials, which generally have percolation thresholdsabove 40%. The relatively low percolation thresholds of the presentdisclosure are achieved by using relatively small particles ofconductive filler in the PPTC material. For example, in variousembodiments, the median diameter of the particles of conductive fillerin the PPTC material may be in a range of about 50 nanometers to 20micrometers. It has been found that using conductive particles of suchrelatively small size can achieve a given resistivity in a PPTC materialusing a smaller quantity of conductive filler by volume relative toparticles of larger size that are traditionally used in conventionalPPTC materials. The cost and weight of the PPTC material of the presentdisclosure may therefore be lower than those of traditional PPTC deviceswhile achieving similar operational characteristics such as resistivityand trip temperature.

Turning now to FIG. 2A, there is shown a graph plotting theresistivities of PPTC materials in accordance with the presentdisclosure as a function of volume fractions of conductive filler(tungsten carbide in this example) of different particle sizes in suchmaterials. As can be seen, a PPTC material having a volume fraction ofabout 27% of conductive particles with a median diameter of 0.55micrometers may have a resistivity of about 0.15 ohm-cm, and a PPTCmaterial having a volume fraction of about 41.2% of conductive particleswith a median diameter of 2.15 micrometers may also have a resistivityof about 0.15 ohm-cm. FIG. 2B shows a bar graph illustrating percolationthresholds of various sizes of conductive filler particles (tungstencarbide in this example) necessary for achieving PPTC materials with thesame resistivity. For example, a PPTC material having conductiveparticles with a medium diameter of 1 micrometer will have a percolationthreshold of about 37% to achieve the same resistivity as a PPTCmaterial having conductive particles with a medium diameter of 1.57micrometers at a percolation threshold of about 39%. Thus, it can beseen that different PPTC materials that include different volumefractions of conductive filler can be made to have similar resistivitiesby varying the sizes of conductive particles in such materials.

Turning now to FIG. 3 there is shown a graph plotting the resistancebehavior as a function of temperature of a PPTC device, arrangedaccording to embodiments of the disclosure. In this example, the PPTCmaterial of the PPTC device has a percolation threshold of 35% ofconductive filler (tungsten carbide in this example) with particleshaving a median diameter of 1.57 micrometers. As shown, an abruptincrease in resistance takes place at 160-165° C. Accordingly, the PPTCmaterial of FIG. 3 may be deemed to exhibit a trip temperature of about163° C.

The hold current density of the PPTC materials of the present disclosuremay be designed to exhibit a value between 0.05 to 0.4 A/mm² byappropriate choice of volume fraction of conductive filler and type ofconductive filler, where hold current density is calculated as a ratioof the hold current of a PPTC material at 25° C. to the area of the PPTCthrough which current travels between opposing electrodes.

The configuration of a PPTC device may vary according to differentembodiments of the present disclosure. FIG. 4 presents a top plan viewof a PPTC device 400, shown as a radial lead PPTC device, including abottom lead 404 and a top lead 406, attached to opposite surfaces of aPPTC body 402. The PPTC body 402 may have first and second electrodes(not separately shown) attached to the top surface and bottom surfacethereof, respectively, as generally described above. The PPTC device 400may be encapsulated by an encapsulant layer 410, such as an epoxy. ThePPTC body 402 may be formed of a PPTC material formulated generally asdescribed above, having low percolations thresholds, such as in a rangeof 20%-39%.

FIG. 5A and FIG. 5B depict side cross-sectional views of embodiments ofa single-layer surface mount PPTC device 500 and a double-layer surfacemount PPTC device 600, respectively, according to exemplary embodimentsof the present disclosure. These devices may include PPTC bodies 502,and first and second metallic foil layers 504 a, 504 b disposed onopposing sides of the PPTC bodies 502 and extending longitudinally fromfirst and second metallic traces 506 a, 506 b at opposing longitudinalends of the PPTC bodies 502, wherein the first metallic foil layers 504extend toward, but does not contact, the second metallic traces 506 b,and wherein the second metallic foil layers 504 b extend toward, butdoes not contact, the first metallic traces 506 a. The devices mayfurther include electrically insulating insulation layers 510 coveringthe metallic foil layers 504 a, 504 b, and metallic electrodes 512disposed on the outermost insulation layers 510 in electrical contactwith the metallic traces 506 a, 506 b. In these devices, the PPTC bodies502 may be formed of a PPTC material formulated generally as describedabove, having a low percolation threshold, such as in a range of20%-39%.

While the present embodiments have been disclosed with reference tocertain embodiments, numerous modifications, alterations and changes tothe described embodiments are possible while not departing from thesphere and scope of the present disclosure, as defined in the appendedclaims. Accordingly, the present embodiments are not to be limited tothe described embodiments and may have the full scope defined by thelanguage of the following claims, and equivalents thereof.

What is claimed is:
 1. A polymeric positive temperature coefficient(PPTC) device, comprising: a PPTC body; a first electrode disposed on afirst side of the PPTC body; and a second electrode disposed on a secondside of the PPTC body; wherein the PPTC body is formed of a PPTCmaterial that includes a polymer matrix and a conductive fillercomprising tungsten carbide, the conductive filler further comprising avolume fraction parameter and a median diameter of conductive particlesparameter such that PPTC material exhibits a resistivity ofapproximately 0.15 ohm-cm; wherein the volume fraction parameter andmedian diameter of conductive particles parameter are selected from agroup consisting of: 27% volume fraction of conductive particles with amedian diameter of 0.55 μm; 37% volume fraction of conductive particleswith a median diameter of 1.0 μm; 39% volume fraction of conductiveparticles with a median diameter of 1.57 μm; 41.2% volume fraction ofconductive particles with a median diameter of 2.15 μm; 42.5% volumefraction of conductive particles with a median diameter of 3.21 μm; and45.5% volume fraction of conductive particles with a median diameter of4.82 μm.
 2. The PPTC device of claim 1, wherein the PPTC materialexhibits a hold current density of between 0.05 to 0.4A/mm².
 3. The PPTCdevice of claim 1, wherein the polymer matrix includes at least one of apolyvinylidene fluoride (PVDF) polymer, an ethylene vinyl acetate (EVA)polymer, a high-density polyethylene (HDPE) polymer, an ethylenetetrafluoroethylene (ETFE) polymer, and a perfluoroalkoxy (PFA).
 4. ThePPTC device of claim 1, wherein at least one of the first electrode andthe second electrode is formed of copper foil.
 5. The PPTC device ofclaim 4, wherein the copper foil is plated with nickel.
 6. A polymericpositive temperature coefficient (PPTC) material comprising: a polymermatrix; and a conductive filler comprising tungsten carbide, theconductive filler further comprising a volume fraction parameter and amedian diameter of conductive particles parameter such that PPTCmaterial exhibits a resistivity of approximately 0.15 ohm-cm; whereinthe volume fraction parameter and median diameter of conductiveparticles parameter are selected from a group consisting of: 27% volumefraction of conductive particles with a median diameter of 0.55 μm; 37%volume fraction of conductive particles with a median diameter of 1.0μm; 39% volume fraction of conductive particles with a median diameterof 1.57 μm; 41.2% volume fraction of conductive particles with a mediandiameter of 2.15 μm; 42.5% volume fraction of conductive particles witha median diameter of 3.21 μm; and 45.5% volume fraction of conductiveparticles with a median diameter of 4.82 μm.
 7. The PPTC material ofclaim 6, wherein the PPTC material exhibits a hold current density ofbetween 0.05 to 0.4A/mm².
 8. The PPTC material of claim 6, wherein thepolymer matrix includes at least one of a polyvinylidene fluoride (PVDF)polymer, an ethylene vinyl acetate (EVA) polymer, a high-densitypolyethylene (HDPE) polymer, an ethylene tetrafluoroethylene (ETFE)polymer, and a perfluoroalkoxy (PFA).